Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius).

Grease and flour a 9x5 inch loaf pan.

In a large bowl, cream together the softened butter and sugar until light and fluffy.

Beat in the eggs one at a time, mixing well after each addition.

Stir in the buttermilk and orange juice until combined.

In a separate bowl, combine the all-purpose flour, oat flour, cocoa powder, baking powder, baking soda, salt, and cinnamon.

Add the orange zest to the flour mixture and coat it well.

Gently fold the dry ingredients into the wet ingredients until just combined. Do not overmix.

Pour the batter into the prepared loaf pan.

Tap the pan gently on the counter a few times to release any air bubbles.

Bake at 350 degrees F (175 degrees C) for 10 minutes.

Reduce the oven temperature to 325 degrees F (160 degrees C) and continue baking for another 30-40 minutes, or until a wooden skewer inserted into the center comes out clean.

If the top of the bread starts to brown too quickly, cover the pan loosely with foil during the last 15-20 minutes of baking.

Once baked, let the bread cool in the pan for 10-15 minutes before transferring it to a wire rack to cool completely before serving.